Castor bean (Ricinus communis) is
a perennial shrub, sometimes tree-like, three to fifteen feet tall, with large,
palmately lobed leaves and sharply toothed leaf margins. The leaves are usually
deep green, but in some strains they have a reddish cast. They have an odor when
crushed. The stems are smooth, round, and frequently red, with clear sap. The
flowers are small and greenish, with both male and female flowers on the same
plant. The fruit is a quarter-sized, round, spiny capsule, often reddish,
containing up to three shiny, smooth, mottled seeds that resemble ticks.

Description:

Euphorbiaceae. Shrub to 15 ft (5 m). Leaves: simple, 4-16 in (10-40 cm) broad, palmately 5-11 lobed with serrate margins. Leaves are alternate on the stem and peltate (petiole attached inside the edge of the blade rather than along the edge as in most species). Petioles with conspicuous glands and stipules fused and sheaf-like. Inflorescence: terminal panicle, 4-12 in (10-30 cm) long. Flowers: plants are monoecious (separate male and female flowers on the same plant).

Male flower has many stamens on
much-branched filaments and a 3-5 parted calyx; female flowers (located above
male flowers) have 3 red styles united at the base and a calyx that falls early.
Styles are bifid, plumose, and red. There are no petals, nectaries, or disks on
flowers of either sex. Fruit: capsule with three 2-valved carpels, 0.4-0.8 in
(1-2 cm) in diameter with soft spines. One seed in each carpel. Seeds glabrous,
shiny, 0.12-0.16 in (3-4 mm) wide, 0.4-0.9 in (9-22 mm) long, and flattish,
oblong-ellipsoid, variously marked and colored more or less mottled
lustrous-silvery and brown with a fleshy appendage at one end (Hickman 1993,
Parsons 1992).

WHERE WOULD I FIND IT?

Castor bean is widespread in the southern
United States, where it has been introduced and naturalized. It is grown as a
cultivated crop in California, Illinois, Missouri, Kansas, Oklahoma, and Oregon.
In California, outside cultivation, castor bean has naturalized below 1,000 feet
(300 m) elevation in the southern San Joaquin Valley, along the central and
south coast, in the San Francisco Bay Area, and in Trinity County. It grows as a
shrub in mild climates such as coastal southern California, but can grow as an
annual in colder climates (Munz 1974).

Castor bean is frequently found in riparian areas, especially
along the south and central coast, where it invades and displaces native
vegetation. This plant is also common as an escape in abandoned fields,
drainages, ditches, and along roadsides and railroad tracks. It is killed by low
temperatures (Robbins et al. 1941), and as little as twenty-four hours at 2
degrees F is sufficient to produce visible impacts on cellular membranes of
seedlings at any stage of germination (Breidenbach et al. 1974). Distribution is
limited by castor bean’s intolerance of cold temperatures and the inhibitory
effect of low humidity or water stress on photosynthesis (Dai et al. 1992). It
is tolerant of a wide range of soil types and conditions. Plants tend to
germinate more profusely in full sun (Kitz, pers. comm.).

WHERE DID IT COME FROM AND HOW IS IT SPREAD?

Castor bean is native to warmer parts of
Asia and Africa (Robbins et al. 1941). This plant has been cultivated as an oil
crop (Whitson 1992) and as an ornamental (Hogan 1992). It has been used as a
purgative and an industrial oil, and seeds are currently sold as a gopher
deterrent. Castor bean spreads by seed and is capable of resprouting from the
root crown if cut. The seed pods dehisce when ripe, spreading seeds near the
parent plant (Parsons 1992). Seeds can be carried to new locations by moving
water or by transport of soil. Seeds also may be spread by road maintenance
machinery. The plant is not known to spread by root fragments.

WHAT PROBLEMS DOES IT CAUSE?

Castor bean displaces native plant species in riparian areas and
drainages. Its seeds are among the first to germinate following fire. Plants
colonize disturbed areas, and they grow rapidly, shading out native seeds and
seedlings and producing monospecific stands in areas with previously healthy
native vegetation.

Castor bean seeds are highly toxic to humans, cattle, horses,
rabbits, sheep, pigs, goats, gophers, cats, dogs, and poultry (Robbins et al.
1941, Cooper and Johnson 1984, Reynolds 1996). Ingestion of two beans can be
lethal to humans, and the toxin and its mode of action have been well
characterized and utilized in cancer treatment (Saelinger 1990). Ricin, the
toxic water-soluble protein that can make castor bean deadly, is at its highest
concentration in the seeds, but is also found in the leaves. The seed coat must
be damaged to allow water to penetrate the seed interior for ricin to be
absorbed in the intestines (Cooper and Johnson 1984). Most reports of animal
deaths are associated with livestock, but several thousand ducks in the Texas
Panhandle reportedly died from ingesting castor beans in fall and winter 1969-71
(Jensen and Allen 1981). Aphids are susceptible to poisoning by ingesting the
phloem, and European corn borer and southern corn rootworm larvae were killed
when exposed to feed painted with 2 percent ricin (Olaifa et al. 1991).
Parasitic soil nematodes have been shown to decrease in number in soil
associated with castor bean plants (Fuller and McClintock 1986).

Castor bean reproduces by seed. Plants become reproductive in the first season (within six months) and are capable of flowering year round in a frost-free environment. A single large plant 10.2 feet (8 m) diameter was found to produce 150,000 seeds, while a smaller plant thirty-nine inches (1 m) diameter produced only 1,500 seeds. Seeds generally germinate in December-April, but depending on weather and soil moisture, plants may germinate at other times of year.
The plant can grow 6.5 feet (2-5 m) in a single season in full sun with plenty of heat and moisture (Hogan 1992). Castor bean is a tropical plant that has a high photosynthetic capacity with high humidity (Dai et al. 1992). Low humidity strongly inhibits photosynthesis because of stomatal closure. Castor bean is not found in arid places. Freezing temperatures kill seedlings, but plants in coastal areas tend to grow and bloom year round as perennials. In areas subject to frost, plants grow as annuals. A long frost-free period is needed for seeds to develop. Shade tends to inhibit germination and produce smaller, slower-growing plants.

Areas of native vegetation subjected to fire have produced solid stands of castor bean, although castor bean plants had been absent from the area for more than ten years, suggesting that seeds of castor bean are long-lived (Kitz, pers. comm.). Plants resprout from root crowns when cut.

(click on photos to view larger image)

HOW CAN I GET RID OF IT?

The best method to remove castor bean
depends on the size of the plant, soil type and moisture, and the importance of
avoiding impacts to non-target species. Follow-up monitoring is always
appropriate. If mature plants containing seeds are not removed from the site,
efforts should be made to remove resulting seedlings.

Physical control:

Mechanical methods: Pulling plants by hand when small or in wet
sandy soils is a feasible technique in most riparian areas. The bulk of the root
should be removed. Plants broken at the root crown will regenerate with multiple
shoots. Weed wrenches can be used to remove small to medium-sized plants. If
soil is so dry that plants break off from the main root, chemical treatment is
needed. Gloves should be worn for hand pulling.

Prescribed burning: Burning is not recommended for castor
bean removal in coastal areas as it creates ideal conditions for habitat
conversion. Seeds in or on the soil readily germinate after fire, and seedlings
grow so rapidly that they outcompete other species, dominating the area and
driving out desirable natives. Studies of the impacts of burns and multiple
burns are not known. In areas subject to frost prescribed burns might be useful
if they are timed appropriately.

Biological control:

Since castor bean is a crop in some places
in the United States, there is no biocontrol program. A large number of diseases
and pests are known to impact castor bean crops (Kranz et al. 1977). Mung moth,
pink bollworm, scab, wilt, leaf spot, seedling blight, inflorescence rot, pod
rot, rust spot, graymold, crown rot, stem canker, leaf blight, bacterial wilt,
and angular leaf spots are known to impact castor bean crops, but are rarely
seen in riparian wildland plants. Leaves rarely show evidence of herbivory,
although occasional leaf browse recently has been seen in the Santa Monica
Mountains (Kitz, pers. comm.). Grazing is not recommended because of the plant’s
toxicity to livestock and other animals.

Chemical control:

Foliar-sprayed 2 percent glyphosate (as
Roundup®) can be used to kill mature shrubs. Foliar spray can impact non-target
species. Cut-stump treatment with loppers or saws and 25 percent glyphosate can
also be used to kill mature shrubs, eliminating collateral damage to non-target
species and reducing herbicide introduced to the system, especially if the plant
is large. Small saws (hand or chain) will be required for larger
plants.